In conventional telephone systems, the line input voltage to a telephone set can vary considerably from one telephone system and one country to another, and the input voltage to any particular set in any given system varies substantially in accordance with transmission line impedance, most notably the distance of a specific telephone set from the central office.
In connection with the development of proposed new electronic telephone sets, preferably sets of universal, as nearly world-wide applicability as is feasible, it is desirable to provide a set that automatically generates a substantially constant speech network tip-to-ring supply voltage and speech network loop current, as will be described in detail in the section "Detailed Description," such as 6 volts.+-.1/4 volt and 25 ma in a typical example, regardless of the input supply voltage to the set.
In conventional American telephone networks (no active devices in the speech network), the impedance of the set is matched (by varistor networks built into the set) to that of the transmission line by drawing relatively high telephone set currents (such as 150 ma) on short loops and relatively low currents (such as 20 ma) on long loops. Such networks are inherently passive (no gain), and cannot boost transmitted signal level or increase receive sensitivity.